(22-10-2010, 02:05 AM)khlai Wrote: Could any one advise me?
When we say the signal is normal , it means that the signal is at red. Normally it appies to a controlled signal.
How about the automatic signal, does it become red after a train passed. Then it will automatically be clear for next train, it becomes green/double yellow. If so, how long it takes to change from red to green/double yellow.
Two somewhat different subjets here!
1. A route is said to be normal when
a) the signaller has cancelled it to force the signal to be at red, AND
b) the signal trackside has actually responded and is displaying danger (not actually proved alight, but certainly not showing a proceed aspect), AND
c) can be treated as actually being able to stop a train (i.e. it has not only just been placed to red but has been at red long enough, compared to the position of any approaching train, for it to be reasonable to expect the train to stop at it)
Therefore it does apply to a controlled signal.
2. An auto signal usually shows as least restrictive aspect as it can, given aspect of signal ahead and any track xection occupancy. The time for which it is at red depends on how long it takes the previous train to travel the length that means that the section from that auto signal to clear the overlap (generally 180m) beyond the following signal. If that signal is itself an auto then that one will clear to yello once the train has travelled a similar length, thus permitting the initial signal to step up to a Green (or in the case of a 4 aspect sequence, to Double Yellow).
The time for any signal therefore depends upon train speed. Non-stop headway calculations assume trains all travelling at constant "headway" speed; stopping headways take into account the braking and acceleration if a station stop or a length of line with lower permissible speed is entailed.